Identification and characterization of a novel yeast gene: the YGP1 gene product is a highly glycosylated secreted protein that is synthesized in response to nutrient limitation.

Nutrient starvation in the yeast Saccharomyces cerevisiae leads to a number of physiological changes that accompany entry into stationary phase. The expression of genes whose products play a role in stress adaptation is regulated in a manner that allows the cell to sense and respond to changing environmental conditions. We have identified a novel yeast gene, YGP1, that displays homology to the sporulation-specific SPS100 gene. The expression of YGP1 is regulated by nutrient availability. The gene is expressed at a basal level during "respiro-fermentative" (logarithmic) growth. When the glucose concentration in the medium falls below 1%, the YGP1 gene is derepressed and the gene product, gp37, is synthesized at levels up to 50-fold above the basal level. The glucose-sensing mechanism is independent of the SNF1 pathway and does not operate when cells are directly shifted to a low glucose concentration. The expression of YGP1 also responds to the depletion of nitrogen and phosphate, indicating a general response to nutrient deprivation. These results suggest that the YGP1 gene product may be involved in cellular adaptations prior to stationary phase and may be a useful marker protein for monitoring early events associated with the stress response.     

Expression of a yeast glycolytic gene is subject to dosage limitation

The Saccharomyces cerevisiae pyruvate kinase-encoding gene (PYK1) has been transformed back into yeast using a derivative of the multicopy vector, pJDB207. High levels of PYK1 expression in these transformants are limited by at least two separate mechanisms. Pyruvate kinase assays and polysome analyses demonstrate that the translation of the PYK1 mRNA is inhibited as its abundance increases. The abundance of the PYK1 mRNA per gene copy also decreases as the copy number of the PYK1 gene increases. This is the first report which demonstrates that a eukaryotic glycolytic gene is subject to dosage limitation at the translational level.     

Aspartokinase II from Bacillus subtilis is degraded in response to nutrient limitation

Aspartokinase II from Bacillus subtilis was shown by immunochemical methods to be regulated by degradation in response to starvation of cells for various nutrients. Ammonium starvation induced the fastest aspartokinase II decline (t1/2 = 65 min), followed by amino acid starvation (t1/2 = 80 min) and glucose limitation (t1/2 = 120 min). Loss of enzyme activity was closely correlated with the disappearance of the alpha subunit; degradation of the beta subunit was somewhat delayed or slower under some conditions. Pulse-chase experiments demonstrated that aspartokinase II was stable during exponential growth; the synthesis of the enzyme rapidly declined in response to nutrient exhaustion. The degradation of aspartokinase II was interrupted by inhibitors of energy production and protein synthesis but was not changed in a mutant lacking a major intracellular protease. Mutants lacking a normal stringent response displayed only a slight decrease in the rate of aspartokinase II degradation, even though aspartate transcarbamylase was degraded more slowly in the same mutant cells. These results indicate that although energy-dependent degradation of biosynthetic enzymes is a general phenomenon in nutrient-starved B. subtilis cells, the degradation of specific enzymes probably involves different pathways.     

Translational efficiency of a non-AUG initiation codon is significantly affected by its sequence context in yeast

Previous studies have shown that translation of mrna for yeast glycyl-tRNA synthetase is alternatively initiated from UUG and a downstream AUG initiation codon. Evidence presented here shows that unlike an AUG initiation codon, efficiency of this non-AUG initiation codon is significantly affected by its sequence context, in particular the nucleotides at positions -3 to -1 relative to the initiation codon. A/A/R (R represents A Or G) and C/G/C appear to be the most and least favorable sequences at these positions, respectively. Mutation of the native context sequence -3 to -1 from AAA to CGC reduced translation initiation from the UUG codon up to 32-fold and resulted in loss of mitochondrial respiration. although an AUG initiation codon is, in general, unresponsive to context changes in yeast, an AAA (-3 to -1) to CGC mutation still reduced its initiating activity up to 8-fold under similar conditions. these results suggest that sequence context is more important for translation initiation in yeast than previously appreciated.     

The adaptive response to alkylation damage. Constitutive expression through a mutation in the coding region of the ada gene

We have shown by genetic mapping, molecular cloning, and DNA sequencing that four Escherichia coli mutants, which express the adaptive response to alkylation damage constitutively, are mutated in the ada gene. All four mutant ada genes have two GC to AT transition mutations in the coding region and encode altered Ada proteins with two amino acid substitutions in the N-terminal domain. E. coli carrying the mutated ada genes on recombinant plasmids overexpressed both the mutated ada gene and the chromosomal alkA gene. This observation indicates that the mutant Ada proteins act as strong positive regulators of the ada and alkA genes in the absence of DNA alkylation. One mutant protein, Ada-11, was shown to be a strong activator of ada gene expression in a cell-free system. An altered pattern of tryptic digestion of the Ada-11 protein compared with the wild-type Ada protein suggested that it has a different conformation. One amino acid substitution, namely methionine residue 126 replaced by isoleucine, occurred in all four mutant Ada proteins, and this mutation alone was sufficient to convert the Ada protein into a strong activator of ada and alkA gene expression in vivo.     

DNA sequence of the coding region of the HLA-B44 gene

An HLA-B44 cDNA clone was identified in a cDNA library constructed from an HLA-B44 homozygous cell line. The DNA sequence was determined and was found to contain the complete coding sequence but for (probably) the three N-terminal codons. Comparisons of the derived amino acid sequence with other HLA-A and -B locus amino acid sequences revealed four HLA-B44-specific substitutions including a new polymorphic site. Regions of strong sequence conservation for HLA-B-locus products were found at the nucleotide and amino acid levels.     

Sex-specific response to nutrient limitation and its effects on female mating success in a gift-giving butterfly

Animals with complex life cycles respond to early food limitation by altering the way resources are allocated in the adult stage. Response to food limitation should differ between males and females, especially in organisms whose mating systems include nutritional nuptial gifts. In these organisms, males are predicted to keep their allocation to reproduction (sperm and nuptial gift production) constant, while females are predicted to sacrifice allocation to reproduction (egg production) since they can compensate by acquiring nuptial gifts when mating. In this study, we investigated how dietary nitrogen limitation during the larval stage affects sex-specific resource allocation in Pieris rapae butterflies. Also, we tested whether nutrient-limited females increased nuptial gift acquisition as a way to compensate for low allocation to reproduction. We found that as predicted females, but not males, sacrifice allocation to reproduction when larval dietary nitrogen is limited. However, females were unable to compensate for this low reproductive allocation by increasing their mating rate to acquire additional gifts. Females reared on low nitrogen diets also reduced wing coloration, a potential signal of female fecundity status. We suggest that female mating frequency is constrained by male mate choice based on females’ wing coloration. This study provides new insights into how larval dietary nitrogen, a key nutritional resource for all herbivores, alters male and female allocation to reproduction as well as to ornamentation.     

Nucleotide sequence of the coding region of the mouse N-myc gene.

A genomic clone for the mouse N-myc gene was isolated and the total nucleotide sequence (4807 bp) of the two coding exons and an intron located between them was determined. The amino acid sequence of the N-myc protein was deduced from the DNA sequence. This protein is composed of 462 amino acids, slightly larger than human and mouse c-myc proteins, and is rich in proline like the c-myc protein. Comparison of the amino acid sequences of the mouse N-myc and c-myc proteins showed that conserved sequences are located in eight regions: four regions are in the N-terminal half of the N-myc protein and are separated from each other by regions poorly homologous to those of the c-myc protein, and the four others are located in the C-terminal half, throughout which certain homology exists. A remarkable sequence containing 13 successive acidic amino acids is present in one of the conserved regions located in the middle of the N-myc protein.